David Cameron is to unveil plans for a DNA database that could one day hold the genetic details of every person in Britain.

The £100m project will see 100,000 patients with cancer and rare diseases having their DNA fully mapped as part of a drive to revolutionise NHS treatment.

The project, to take place in England over the next three to five years, is aimed at saving thousands of lives - but there are concerns that it could pose risks to privacy.

The Prime Minister said he wanted to "push the boundaries" by introducing mainstream genetic sequencing for the first time.

"Britain has often led the world in scientific breakthroughs and medical innovations, from the first CT scan and test-tube baby through to decoding DNA," Mr Cameron said.

"By unlocking the power of DNA data, the NHS will lead the global race for better tests, better drugs and above all better care.X-ray images of prostate cancer Doctors hope the database will help reduce cancer deaths

"We are turning an important scientific breakthrough into a potentially life-saving reality for NHS patients across the country.

"If we get this right, we could transform how we diagnose and treat our most complex diseases not only here but across the world, while enabling our best scientists to discover the next wonder drug or breakthrough technology."

Some critics of the project, known as the "UK genome plan", have voiced concerns about how the data will be used and shared with third parties, including with commercial organisations such as drug companies.

GeneWatch, a campaign group fighting for genetic science and technologies to be used in the public interest, has said anyone with access to the database could use the genetic codes to identify and track every individual on it and their relatives.

But Downing Street stressed that the genome sequencing would be entirely voluntary and patients will be able to opt out without affecting their NHS care. It added the data would be "completely anonymised before it is stored".

The Government's Chief Medical Officer, Professor Dame Sally Davies, said the NHS already analysed single genes in cancer tests and to check patients' risk of suffering side effects from treatment.

"At the moment, these tests focus on diseases caused by changes in a single gene," she said.

"This funding opens up the possibility of being able to look at the three billion DNA pieces in each of us so we can get a greater understanding of the complex relationship between our genes and lifestyle."

The £100m, which comes from existing NHS budgets, will go on training genetic scientists, mapping patients' DNA, and creating systems for handling the information.

The project that first sequenced a human genome in 2000 cost around £500m, but technical advan